The present invention relates to a power amplifier which is used, for example, in a transmission circuit of a radio communication apparatus and is required particularly to perform a low-distortion amplification, and also relates to a radio communication apparatus using the power amplifier.
In recent years, radio communication systems using a microwave band or a submicrowave band, including mobile communication systems such as a mobile phone, have rapidly come into wide use. This is greatly attributed to the reduction in weight and power consumption of the portable terminals. To make the portable terminal lightweight, it is effective to reduce the weight and capacity of a battery. But the battery having a small capacity is exhausted in a short time. Thus there has been strong demand for the reduction of electric power consumed by a signal transmission power amplifier in the portable terminal, because the power amplifier consumes most of the electric power of the portable terminal. That is, improvement of electric power efficiency has been strongly demanded for. In the recent digital modulation and demodulation using QPSK (Quadrature Phase Shift Keying) modulation technique, the power amplifier is required to amplify an inputted signal linearly because information is represented by amplitude and phase of the signal.
Generally, in the power amplifier, as an output electric power becomes closer to saturation owing to increase of an inputted electric power, distortion and power efficiency become increasingly high. Thus a high power efficiency and a low distortion are in a reciprocal relationship.
A power amplifier developed to realize a low distortion operation is disclosed in Japanese Patent Application Laid-Open No.10-135750
FIG. 13 is a circuit diagram showing the construction of the power amplifier. The power amplifier includes a front-stage signal-amplifying bipolar transistor 101; a rear-stage signal-amplifying bipolar transistor 102, an input terminal 103, an output terminal 104, an input matching circuit 105, a interstage matching circuit 106, an output matching circuit 107, a supply voltage terminal 108, a supply voltage terminal 109 for the base of the front-stage signal-amplifying bipolar transistor 101, and a supply voltage terminal 110 for the base of the rear-stage signal-amplifying bipolar transistor 102.
The operation of the power amplifier having the construction will be described below.
In the power amplifier, the low distortion operation and the high efficiency operation are in the reciprocal relationship, as descried above. To realize both the low distortion operation and the high efficiency operation, a front-stage base-biasing point is set to class A or class AB close to class A, and a rear-stage base-biasing point is set to class B or class AB close to class B.
The operation of the power amplifier will be described below from the viewpoint of a gain distortion of the power amplifier. Because the front stage is set to class A or class AB close to class A, if an input power increases and approaches a saturated power, the gain decreases. On the other hand, because the rear stage is set to class B or class AB close to class B, if the level of the input power increases, the gain also increases. As described above, the amplitude distortion characteristic at the front amplification stage is reverse to that at the rear amplification stage. By controlling the front- and rear-stage amplitude distortion characteristics appropriately, it is possible to adjust the distortion characteristic of the entire power amplifier while not affecting the power efficiency. The distortion characteristic of each of the front and rear stages of the power amplifier can be finely adjusted by changing the base-biasing point of each stage. In particular, because the rear stage is set to class B or class AB close to class B, there is a large change in the distortion characteristic with respect to the change of the base-biasing point. Accordingly, the distortion characteristic of the entire power amplifier is adjusted by utilizing the rear stage mainly.
However, the power amplifier has a problem that because the distortion characteristic is adjusted by changing the base-biasing point, the gain changes simultaneously with the adjustment of the distortion.
Therefore, it is an object of the present invention to provide a high-efficiency and low-distortion power amplifier capable of accomplishing a distortion adjustment with a simple construction, without changing the gain, and also to provide a radio communication apparatus using the power amplifier.
In order to accomplish the above object, a power amplifier according to an aspect of the present invention includes:
a plurality of amplification stages in each of which a transistor is used as an amplification element; and
a distortion compensation circuit for compensating a distortion in at least one of the plurality of amplification stages,
wherein the amplification stage that undergoes distortion compensation by the distortion compensation circuit and the remaining amplification stage or stages that dos not undergo the distortion compensation by the distortion compensation circuit have opposite dependencies of an amplitude and/or a phase on an input power level.
In the power amplifier having the construction, the base-biasing point may be set to, for example, class A or class AB close to class A in the amplification stage that does not undergo the distortion compensation by the distortion compensation circuit, whereas the base-biasing point may be set to, for example, class B or class AB close to class B in the at least one amplification stage that undergoes the distortion compensation by the distortion compensation. The distortion characteristic of the power amplifier will be described below from the viewpoint of the amplitude distortion. The amplification stage not having the distortion compensation circuit has a characteristic that as the input power level increases toward a saturated power, a gain decreases, since its operation is set to class A or class AB close to class A. On the other hand, the amplification stage having the distortion compensation circuit has a characteristic that as the input power level increases toward the a saturated power, the gain increases, since its operation is set to class B or class AB close to class B. That is, the amplification stage having the distortion compensation circuit and the amplification stage not having the distortion compensation circuit have opposite dependencies of the amplitude on the input power level. Accordingly, by optimally adjusting the amplitude distortion characteristic of the amplification stage having the distortion compensation circuit, it is possible to offset the amplitude distortion of the amplification stage not having the distortion compensation circuit. If the amplification stage having the distortion compensation circuit and the amplification stage not having the distortion compensation circuit have opposite dependencies of the phase on the input power level, it is possible to adjust the phase distortion characteristic of the amplification stage having the distortion compensation circuit in such a way as to offset the phase distortion of the amplification stage not having the distortion compensation circuit. The transistor in each amplification stage may be a bipolar transistor or a field effect transistor.
There is also provided a power amplifier, according to another aspect of the present invention, which includes:
a plurality of amplification stages in each of which a bipolar transistor is used as an amplification element; and
a distortion compensation circuit for compensating a distortion in at least one of the plurality of amplification stages,
the distortion compensation circuit functioning also as a biasing circuit for the bipolar transistor in the amplification stage that undergoes distortion compensation by the distortion compensation circuit.
In the power amplifier having the above construction, by compensating at least one of the amplitude or the phase in the at least one amplification stage by the distortion compensation circuit, the distortion characteristic of the entire power amplifier is compensated. For example, one or more amplification stages (or all amplification stages) may be provided with the distortion compensation circuit so that each distortion compensation circuit compensates the distortion characteristic of the entire power amplifier. Further, because the distortion compensation circuit serves also as a biasing circuit for the bipolar transistor of the amplification stage that undergoes the distortion compensation, it is unnecessary to additionally provide such a biasing circuit, and the insertion of the distortion compensation circuit does not cause the deterioration of the characteristic of the power amplifier and the increase of the chip area.
A power amplifier according to still another aspect of the present invention includes:
a plurality of amplification stages in each of which a bipolar transistor is used as an amplification element;
a voltage application means for applying a voltage to a base of each of the bipolar transistors in the amplification stages; and
a distortion compensation circuit for compensating a distortion in at least one of the amplification stages,
the distortion compensation circuit including a variable impedance element connected between the voltage application means and the base of the bipolar transistor in the amplification stage that undergoes distortion compensation by the distortion compensation circuit.
In the power amplifier having the construction, the variable impedance element whose impedance or DC resistance component changes with the increase of the level of the input power is used for the distortion compensation circuit to change the transmission characteristic of the distortion compensation circuit owing to the change in the impedance of the variable impedance element and change the operation class with the increase of the level of the input power. Consequently, according to the level of the input power, it is possible to give an influence on the distortion characteristic (amplitude distortion, phase distortion) of the amplification stage having the distortion compensation circuit. For example, when the distortion characteristic is considered in terms of the distortion of the gain, it is possible to increase or decrease the amount of change of the gain without changing the operation class where there is no signal. Accordingly, it is possible to optimally adjust the distortion characteristic of the amplification stage having the distortion compensation circuit, without deteriorating the gain, in such a way as to offset the distortion of the amplification stage not having the distortion compensation circuit.
In one embodiment, the distortion compensation circuit includes an adjusting circuit for adjusting a reactance component and/or a resistance component of an impedance characteristic when the variable impedance element is viewed from the bipolar transistor of the amplification stage that undergoes distortion compensation by the distortion compensation circuit.
In this embodiment, the transmission characteristic of the distortion compensation circuit is adjustable by the adjusting circuit connected to the variable impedance element.
In one embodiment, the distortion compensation circuit includes a DC current adjusting element for adjusting a bias current flowing through a base of the bipolar transistor of the amplification stage that undergoes distortion compensation by the distortion compensation circuit.
In this embodiment, the DC current adjusting element is capable of adjusting the change of the operation class of the amplification stage that undergoes the distortion compensation performed by the distortion compensation circuit.
In one embodiment, the adjusting circuit includes a first resistor element having one end connected to a node between the DC current adjusting element and the variable impedance element, and a capacitor connected between the other end of the first resistor element and a ground. Also, the DC current adjusting element includes a second resistor element whose one end is connected to the voltage application means and whose other end is connected to the variable impedance element.
In the embodiment, the value of the impedance or the DC resistance component of the variable impedance element can be easily changed by adjusting the resistance value of the first resistor element and the capacitance value of the capacitor element. Also, the operation class of the amplification stage whose distortion characteristic is corrected by the distortion compensation circuit is easily changed by adjusting the resistance value of the second resistor element.
In one embodiment, the variable impedance element consists of a diode whose anode is connected to the DC current adjusting element and whose cathode is connected to a base of the bipolar transistor in the amplification stage that undergoes the distortion compensation.
In the embodiment, if the diode constituting the variable impedance element is biased in a forward direction, the equivalent resistance value of the diode decreases with the increase of electric current flowing through the diode. On the other hand, as the level of the input power increases, electric current flowing through the base of the bipolar transistor of the amplification stage undergoing the distortion compensation also increases. Accordingly, the higher the level of the input power, the smaller the equivalent resistance value of the diode. Thus, it is possible to change the operation class of the amplification stage that undergoes the distortion compensation with the increase of the level of the input power.
In one embodiment, the variable impedance element is constituted of a bipolar transistor whose emitter is connected to a base of the bipolar transistor in the amplification stage that undergoes the distortion compensation, and whose base is connected to the DC current adjusting element. Thus, the characteristic of dependence upon the output power of the voltage across the variable impedance element is the same as that of the base-emitter voltage of the bipolar transistor in the amplification stage. Thus the arrangement of this embodiment is effective for achieving the distortion compensation for a wide range of input power levels.
In one embodiment, the variable impedance element is constituted of a variable impedance bipolar transistor whose emitter is connected to a base of the bipolar transistor in the amplification stage that undergoes the distortion compensation, and whose base is connected to the DC current adjusting element. The adjusting circuit is constituted of a first resistor element connected between a collector and a base of the variable impedance bipolar transistor and a capacitor element connected between the base of the variable impedance bipolar transistor and a ground. And, the DC current adjusting element is constituted of a second resistor element whose one end is connected to the voltage application means and whose other end is connected to the variable impedance element.
In the power amplifier of the embodiment, a voltage is also applied across the collector and the emitter of the variable impedance bipolar transistor serving as the variable impedance element. Because that voltage is adjustable by the first resistor element, it is possible to adjust the characteristic of the variable impedance element.
In one embodiment, the variable impedance element is constituted of a variable impedance bipolar transistor whose emitter is connected to a base of the bipolar transistor in the amplification stage that undergoes the distortion compensation, and whose base is connected to the DC current adjusting element. The adjusting circuit is constituted of a first resistor element having one end connected to a node between the DC current adjusting element and the variable impedance element, and a capacitor connected between the other end of the first resistor element and a ground. Also, the DC current adjusting element is constituted of a second resistor element whose one end is connected to the voltage application means and whose other end is connected to the variable impedance element, and a third resistor element connected between the base of the bipolar transistor in the amplification stage that undergoes the distortion compensation and a ground.
In the embodiment, the base-biasing point of the bipolar transistor in the amplification stage that undergoes the distortion compensation is determined by the ratio of the resistance values of the second and third resistor elements. Therefore, it is possible to adjust a base bias voltage by the third resistor element after having optimally adjusted the distortion characteristic by the second resistor element.
There is also provided a radio communication apparatus, according to a further aspect of the present invention, that uses the power amplifier of any type described above in a functional block for supplying a signal to a transmission antenna thereof.
Use of the power amplifier in the radio communication apparatus provides the latter with a characteristic of high efficiency and low distortion when the radio communication apparatus transmits a signal. Thus, it is possible to design the radio communication apparatus so as to be compact.